It is known that an aqueous solution containing fine solid particles with a size or less than 10 .mu.m in suspention is treated by adding a flocculant to produce relatively large secondary particles, which are then separated as precipitated matters or as floating matters from the aqueous solution.
This known method, however, entails a poor efficiency and high cost because of the use of flocculant chemicals, and is accompanied problems of being contaminated the aqueous solution and fine particles by excessive flocculant chemicals that may remain within them after the treatment since the use of excessive amount of the flocculant is inevitable when the concentration of the fine particles is as dilute as less than 100 ppm.
The treated solution may additionally be subjected to pass through a layer of natural sand in order to remove remaining fine particles. However, the sand layer of natural silicates are not effective in removing fine particles and, moreover, a mesh of the sand layer is easily be clogged in this filtration.
It is also known to pass an aqueous solution containing fine particles directly through a fine mesh web-like filter produced by hollow fiber in order to remove fine particles with the size of less than 10 .mu.m. With this second known method, however, the filter can be easily clogged unless frequent cleaning by back flowing water, which results in a large volume of back flowing water and cumbersome process.
It is also known a method of treating an aqueous solution containing fine particles with a size of less than 10 .mu.m in suspension to use a type of flocculant called "emulsion breaker" in order to flocculate fine particles and to float on the surface of the solution, which are then collected using a surface-hydrophobic absorbent. Air may be blown into the solution to make the flocculates to float on the surface of the solution. This method is, however, accompanied problem of being contaminated the solution by the excessive flocculant that may remain in the solution after the treatment particulary when the concentration of fine particle is 1.about.100 ppm. Besides, the flocculant and the absorbent are relatively expensive to pushup the overall cost of the treatment.
As described above, any known methods are not satisfactorily effective and relatively high cost particularly when the size of fine particles is less than 10 .mu.m and when the concentration of the fine particles in aqueous solution is as dilute as 1.about.100 ppm.